Direct Carbonization of Cyanopyridinium Crystalline Dicationic Salts

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Direct Carbonization of Cyanopyridinium Crystalline Dicationic Salts into Nitrogen-Enriched Ultra-Microporous Carbons toward Excellent CO2 Adsorption Guojian Chen, Xiaochen Wang, Jing Li, Wei Hou, Yu Zhou,* and Jun Wang* State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing Tech University, Nanjing 210009, P. R. China S Supporting Information *

ABSTRACT: A family of nitrogen-enriched ultramicroporous carbon materials was prepared by direct carbonization of taskspecifically designed molecular carbon precursors of cyanopyridinium-based crystalline dicationic salts (CISs). Varying the molecular structure of CISs, large surface area (918 m2 g−1), high N content (20.10 wt %), and narrow distributed ultramicropores (0.59 nm) can be simultaneously achieved on the sample PCN-14 derived from methyl-linked 4cyanopyridinium D[4-CNPyMe]Tf2N. It therefore exhibited exceptional performance in greenhouse gas CO2 capture, i.e., simultaneously possessing (1) high CO2 adsorption uptakes: 5.33 mmol g−1 at 273 K, and 3.68 mmol g−1 at 298 K (both at 1.0 bar); (2) unprecedented selectivity of CO2 versus N2: 156; and (3) a high adsorption ratio of CO2 to N2: 148 (at 1.0 bar). This is the first time such a high selectivity and adsorption ratio over carbon materials has been achieved, which is among the highest values over solid adsorbents. KEYWORDS: CO2 capture, microporous materials, porous carbon, ionic liquids, molecular precursors

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INTRODUCTION Nanoporous carbon materials attract increasing attention and exhibit diverse applications in energy,1 the environment,2 catalysis,3 adsorption, and separation,4 because of the large surface area, high chemical and physical stability, tailored pore architectures, and surface properties.5 Various porous carbons with multiscale pore structures have been specifically designed, including microporous, ordered/disordered mesoporous, macroporous, and hierarchical porous carbons.6,7 It has been an increasingly hot topic to task-specifically and facilely design new carbon materials with high performances. Among various strategies to achieve carbon materials,8−14 direct carbonization is demonstrated to be very efficient due to the avoidance of any templates or post-treatments, motivating the development of various promising carbon precursors, such as microporous organic polymers (MOPs),15−17 metal−organic frameworks (MOFs),18,19 biomass and derivatives,20 and ionic liquids (ILs).21 However, it is still a very difficult issue to precisely control the synthesis of the desirable porous carbon materials for specific applications. The sharply rising level of CO2 in the atmosphere has urged the development of effective CO2 capture and sequestration technologies to reduce the emissions of greenhouse gases.22 Current CO2 adsorption and separation are dominated by amine-based absorption regeneration processes, limited by energy-cost, corrosion, and inefficiency.23 As a result, a wide © XXXX American Chemical Society

variety of alternative porous solid adsorbents have been prepared and used in CO2 capture for high adsorption capacity and good regeneration stability.24−29 Among them, porous carbons are believed to be one of the most promising candidates for CO2 capture due to their various advantageous internal properties, as mentioned before.5,29 Most commercial activated carbons exhibit moderate CO2 uptakes due to the weak physisorption in undecorated carbon frameworks.30 Therefore, much effort has been proposed to prepare porous carbon materials with high CO2 adsorption capacity.13−20 However, these carbons still suffered from inferior selectivity (e.g., CO2/N2 selectivity ranging from 18 to 79)17,18,29,31 compared to those of microporous organic polymers (MOPs, CO2/N2 selectivity >100),32,33 in spite of the higher CO2 adsorption capacity over the former rather than the latter. Besides, the CO2/N2 selectivity over carbons dropped rapidly with the increase of pressure (e.g., up to 1.0 bar), giving a much lower adsorption ratio of CO2 to N2. The reason can be assigned to the essential physical adsorption property that cannot provide enough CO2-affinity and N2-phobic at high pressure. The above-described phenomena can also be observed over those MOPs with high CO2/N2 selectivities. Received: June 2, 2015 Accepted: July 30, 2015

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DOI: 10.1021/acsami.5b04842 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Scheme 1. Preparation of the Typical N-Enriched Ultra-Microporous Carbon PCN-14 Directly Carbonized from the Newly Designed Cyanopyridinium Crystalline Dicationic Salt (CIS) Precursor D[4-CNPyMe]Tf2N

typical carbon sample of PCN-14, enabling excellent CO2 capture performances: (i) very high CO2 uptakes (5.33 mmol g−1 at 273 K, 1.0 bar and 3.68 mmol g−1 at 298 K, 1.0 bar); (ii) unprecedented CO2/N2 selectivity (156 at 273 K); and (iii) the high adsorption ratio of CO2 to N2 (148 at 273 K, 1.0 bar). This work breaks through the inferior CO2/N2 selectivity over previous porous carbons, while also giving a high CO2 adsorption capacity.

Presently, it remains a huge challenge to simultaneously achieve excellent CO2/N2 selectivity with high CO2 adsorption capacity. Previous works have pointed out that CO2 capture over carbon is closely related to its porosity and surface chemical state. Current strategies toward high-performance carbons focus on the modification of surface compositions (e.g., the doping of heteroatoms) and pore structure control.34−43 Incorporating heteroatoms such as nitrogen (N) into the carbon matrix has been demonstrated as a remarkable strategy to enhance CO2 capture by virtue of the improved affinity interaction between acidic CO2 molecules and N active basic sites.34−39 Besides, it has also been indicated that the rich and narrowly distributed micropores smaller than 1.0 nm favor both CO2 adsorption capacity and selectivity.40−43 However, no such carbon material containing both high N content and strictly ultramicroporous structure has appeared, which may account for the current inferior selective capture capacity of CO2. In this work, we develop a new family of molecular level carbon precursors, i.e., cyanopyridinium crystalline dicationic salts (named as CISs), which are directly carbonized to fabricate N-enriched ultramicroporous carbon series (PCNs) at a relatively low temperature of 450 °C without any additives or pre/post-treatments (Scheme 1). As is known, molecular carbon precursors benefit from finely controlling the composition and pore structure of the target carbon materials, but rarely can they be directly carbonized to porous carbons.44 Some special ILs with cross-linkable groups (e.g., cyano group) or protic ionic liquids/salts were promising carbon precursors for directly producing micro/mesoporous carbons with large surface areas and high N contents.45−49 Those IL-derived carbons showed moderate CO2 uptakes of 2.0−3.0 mmol g−1 (298 K and 1.0 bar) and low CO2/N2 selectivities of 20− 37.47,49 Herein, the molecular precursors of CISs are taskspecifically designed by quaternization reaction of 4-/3cyanopyridine with dibromomethane with successive anionexchange of the commercially available bulky anion [Tf2N] (bis(trifluoromethanesulfonyl)imide). These CISs have similar structures to common ILs but own well-defined crystalline structures and high melting points (most of them >140 °C). Detailed systemic study is performed over the carbonization process, revealing that the porosities and N contents of the obtained CISs-derived PCNs can be tailored by elaborately adjusting alkyl and alkylaryl (i.e., methyl, ethyl, butyl, xylyl, and dixylyl) halide organic linkers and the positions of cyano groups on pyridine rings. The intrinsic cyano groups fasten to pyridine rings, and the bulky anions play key roles in the formation of the strict ultramicropores (10 μm) is aggregated from very small intertwined nanoparticles sized at 10−20 nm without observing mesopores (Figure 2b and Figure S15), except for the sample PCN-54 D


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Figure 4. High resolution XPS spectra of N-enriched carbon PCN-14 prepared at 450 °C: (a) Survey, (b) C 1s, (c) N 1s, and (d) S 2p. (e) 13C CP/ MAS NMR spectrum. (f) Raman spectrum.

pyridine rings can affect the polymerization of cyano groups and the carbonization process, and thus the more symmetric 4CNPy-based CISs make for the formation of PCN products with higher surface areas. The N contents and chemical states of the C, N, and S atoms for the selected sample PCN-14 are further evaluated by XPS, NMR, and Raman spectra. The XPS survey spectrum of PCN14 (Figure 4a) presents five dominant peaks centered at 284.6, 398.3, 532.0, 688.0, and 167.9 eV, corresponding to the presence of C 1s, N 1s, O 1s, F 1s, and S 2p, respectively. The N content value of PCN-14 detected by XPS is 15.95 at% (16.84 wt %) on the surface, which is not far from the one (20.10 wt %) in the bulk carbon tested by elemental analysis, reflecting a relative homogeneous dispersion of the N atom, as demonstrated by the EDS elemental mapping analysis (Figure 2d). Figure 4, parts b and c, depicts the C 1s and N 1s corelevel spectra, respectively. Fitting the C 1s spectrum with multiple peaks reveals a dominant peak at 284.4 eV corresponding to sp2-hybridized graphitic carbon atoms and the other three peaks attributable to CN (285.8 eV), pyridinic carbon CN (287.6 eV), and CSO (291.6 eV), indicating that most of the carbon atoms are arranged in a conjugated honeycomb lattice.49 The N 1s spectrum indicates that the N species mainly consist of pyridinic N (398.3 eV) and pyrrolic N (400.3 eV) originating from the pyridine and triazine rings, which are beneficial to CO2 capture;51 and a minor amount of oxidized N species is observed at higher binding energies (404.5 eV).52 The S 2p spectrum shows the S species (3.25 wt %) doped in PCN-14, in agreement with the elemental analysis result (3.06 wt %, Table S1). The S 2p peak (Figure 3d) can be resolved into two peaks: the one located at 163.9 eV corresponds to neutral thiophene S and the other one located at 167.9 eV, attributable to oxidized S species from the [Tf2N] anion.49 The solid-state 13C CP/MAS NMR spectrum of PCN14 gives two apparent signals for the formed N-doped graphitic carbon (Figure 4e). The peak at δ = 125.5 ppm is typical for all-

ity over these samples, in good accord with the compact morphologies observed in SEM images. One exception is the sample PCN-54 derived from xylyl-linked CIS precursor. Its N2 sorption isotherm is type IV with an H1 hysteresis loop at P/P0 = 0.80−0.99, indicating the presence of abundant mesopores originating from the nanovoids among the aggregated nanoparticles shown in the SEM image (Figure S15). The mesoporous aggregate architecture of PCN-54 can be associated with its specific molecular structure and distorted steric configuration with the rigid aromatic xylyl linker.46,47 Figure 3b gives the micropore size distribution curves calculated by the Horvath−Kawazoe (H−K) method. All these PCN-xy samples demonstrate very narrow pore size distributions centered below 0.7 nm with a half peak width smaller than 0.03 nm. Such special porosity can be assigned to the ultramicroporous structure characteristic of small pore sizes lower than 0.7 nm, which is confirmed by the TEM images (Figure 2, parts e and f), which therefore implies the fabrication of N-enriched ultramicroporous carbon materials by direct carbonization of the molecular CISs. The textural properties of the PCN-xy series (Table 1) reveal that they all possess high surface areas (672−1040 m2 g−1) and large pore volumes. The exact porous structure, including the surface area, pore volume, and pore size of the obtained carbon materials, is also related to the molecular structures of CIS precursors (the length of carbon chains for organic linkers and the position of the CN group on pyridine rings). For alkyl chain linked CISs, shorter carbon chains yield higher surface areas for the final carbon materials; and the CN group at the 4 position of the pyridine ring also produces a higher surface area than the one at the 3 position of the pyridine ring. The situation is different using xylyl- or dixylyl-linked CISs. The sample PCN54 with the rigid xylyl linker is a micro/mesoporous carbon material, while PCN-64 with the dixylyl linker possesses the highest surface area of 1040 m2 g−1 and an ultrasmall micropore size of 0.51 nm. The steric configuration of CN groups on the E


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Figure 5. (a) N2 sorption isotherms of N-enriched carbons PCN-14(T). (b) Micropore size distribution of PCN-14(T) series calculated by the H− K method. (c) BET surface areas (SBET) values and (d) C, N, and S contents of PCN-14(T) as a function of the carbonization temperature. (e) Morphological variation and gas release process during the formation of PCN-14.

carbon-bonded aromatic carbons (CC), while the peak at δ = 150.1 ppm is attributed to aromatic carbon bound to nitrogen atom (CN).50−52 The other weak board peaks (read asterisks) are assigned to the few residual CIS cations and [Tf2N] anions.51 The Raman spectrum of PCN-14 (Figure 4f) shows two intensive bands at 1550 (G band) and 1371 cm−1 (D band) featured for the ideal graphitic sp2 carbons and disordered carbons, respectively.53 Additionally, a broad intensity band at 2765 cm−1 (2D + 2G) is related to the defected carbons. The above analyses demonstrate the formation of N-enriched ultramicroporous carbon materials with adjustable high surface area and pore structure determined by the molecular structure of CIS precursors. The results not only give rise to a promising porous solid adsorbent for selective CO2 capture, but also provide a good opportunity to discuss the structure−activity relationship for subsequent applications. Pore Formation Process. The formation process of carbon materials is studied by carbonizing D[4-CNPyMe]Tf2N, the precursor for preparing PCN-14, at different temperatures. The TGA result (Figure S11) indicates that D[4-CNPyMe]Tf2N begins to decompose at ca. 250 °C, losses 80% weight at 300 °C, and mainly completes the carbonization process at 450 °C. A series of N-enriched carbons named as PCN-14(T) are

prepared with the carbonization temperature (T) from 350 to 600 °C and fully characterized by the N2 sorption (Figure 5a), XRD (Figure S13), FT-IR (Figure S17), and SEM analyses (Figure S18 and Figure 5e). Table S1 lists the textural properties of these samples. With the low carbonization temperature of 350 °C, the obtained sample of PCN-14(350) is nonporous carbon (Table S1) with the morphology of micrometer scaled bulk blocks (Figure S18a). The XRD pattern shows that the crystalline structure of the precursor has disappeared at 350 °C, causing two weak broad peaks at about 21° and 33°, indicating its amorphous structure. As reported by previous work,54 the cyano group in CIS should have undergone a trimerization reaction at 350 °C, which is represented by the appearance of a strong signal at 1351 cm−1 for the formation of triazine rings (FT-IR, Figure S17). The formed triazine-based polymeric cationic network is nonporous because the counterion [Tf2N] still remains trapped within the polymeric network,47,55 in agreement with the high S content of 14.84 wt % (Figure 5d and Table S1). Heating the nonporous polymeric network at a higher temperature than 400 °C causes a dramatic morphological variation to interconnected submicron particles of 100−500 nm (Figure S18). The enhanced XRD signals are observed with slight variation of the peak position, but only a small surface F


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Figure 6. CO2 sorption isotherms of PCN-xy samples at 273 K (a) and 298 K (b) under ambient pressure (1.0 bar). (c) CO2/N2 selectivities of PCN-xy series at 273 K, 1.0 bar. (d) Isosteric heats (Qst) of CO2 adsorption for PCN-xy series calculated using the Clausius−Clapeyron equation.

that the formation of N-enriched ultramicroporous carbon material begins at 440 °C, with a considerable surface area of 433 m2 g−1 and a pore volume of 0.188 cm3 g−1. The large surface area of 918 m2 g−1 (Figure 5c) and pore volume of 0.368 cm3 g−1 are obtained at the higher temperature of 450 °C with the sufficient decomposition of the anions and the release of as-formed gases. Accordingly, the morphology changes to be compact blocks with observations of smooth surface (Figure 5e) and enhanced XRD peaks (Figure S13). All these results together with other characterizations for PCN-14 strongly confirm the formation of N-enriched ultramicroporous carbon material with large surface area and high N content. Furthermore, a higher carbonization temperature up to 600 °C causes a slight variation of FT-IR and morphology results. The surface area and pore volume gradually decrease with rising temperature (Table S1), accompanied with a certain enhancement of XRD peaks and slight decrease of N content and constant S content (Figure 5d). The above survey of the carbonization temperature indicates that the large-surface-area ultramicroporous structure with high N content (20.10 wt %) and S content (3.06 wt %) indeed forms at ca. 450 °C. A polytriazine cationic network forms through trimerization at a relatively low temperature, ca. 350 °C. Along with the carbonization process, ultramicropores originate from the decomposition of anions and the release of as-formed gases CFx and SOx, which happens in a very sharp temperature range, 440−450 °C (Figure 5c), indicating the key role of the counterion as a “molecular porogen”.45,46 The carbonization of D[4-CNPyMe]Br with Br anion fails to produce any porous structure with the same approach, providing an indirect proof that the [Tf2N] anion acts as a molecular template.55 Another precursor D[PyMe]Tf2N without a cross-linkable cyano group results in a very low yield for

area is obtained (Figure 5c). The FT-IR adsorption bands (Figure S17) for CN groups (2254 cm−1), pyridine rings (C H, 3084 cm−1; CN, 1641 cm−1), triazine rings (1351 cm−1), CH2 (2856, 2929, and 1466 cm−1), and Tf2N anion (CF, SO, 1180, 1134, 1049 cm−1) become weakened or disappear, indicating the destruction of the polymeric network and decomposition of F- and S-containing bulky anion [Tf2N]. The C, H, N, and S contents all increase (Figure 5d), suggesting that the release of the as-formed gases mainly related to CFx (i.e., CF4 and C2F6).56 The above results indicate that the carbonization temperature up to 400 °C leads to the partial decomposition of the nonporous polymeric framework formed in a trimerization reaction, but the preservation of S species (15.84 wt %, Figure 5d), still forming the nonporous structure. Increasing the carbonization temperature up to 430 °C causes a continuous decline of the S content and the slight increase of N content, while the C and H elements remain constant (Figure 5d), suggesting the release of a large amount of SOx due to the decomposition of anions.47,55 No obvious variation is observed from the SEM, FT-IR, and XRD. The slight shift of the XRD peaks to higher angles at 430 °C implies the gradual evolution to a graphitic phase. Superior surface area is achieved at 440 °C, accompanied by the further decline of S content (3.23 wt %) and the increase of C and N contents (Figure 5d). SEM images show that the morphology changes to be a compact block, the surface of which still exhibits the aggregation of interconnected particles but with a rough flatlike cross-section. An additional shift to higher angles at 25.6° for (002) peak and 43.0° for (100) peak is observed on the XRD pattern (Figure S13), revealing the formation of a graphitic phase carbon.49 The N2 sorption isotherm is type I, with a narrow pore size distribution centered at 0.61 nm (Figure 5, parts a and b). All of the above phenomena imply G


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∼0.036 mmol g−1 (273 K, 1.0 bar). Correspondingly, the CO2/ N2 adsorption ratio at 1.0 bar is as high as 148 over PCN-14, superior to previous reports (Table S4).62 The ideal adsorbed solution theory (IAST) method has also been applied to determine the CO2/N2 adsorption selectivity of PCN-14 simulated from flue gas mixture (15 wt % CO2/N2). The IAST selectivities of CO2/N2 of PCN-14 and PCN-13 are estimated to be 142 and 121 at 273 K, 1.0 bar (Figure S21), respectively, which are in reasonable agreement with the values obtained from Henry’s constant ratios. Table S4 compares the CO2/N2 adsorption selectivities for PCN-14 and PCN-13 with various other porous solid adsorbents. The comparison indicates that the CO2/N2 selectivity over PCN-14 is superior to the values of all the reported porous carbons, including templated carbons,9,10,35,37 chemical activated carbons,38,40,62 and IL-, MOP- or MOF-derived carbons,15,16,18,31,47,59 even higher than those of the recently reported highest values over porous organic polymers, e.g., electron-rich oganoniridic framework PECONF-1 (109),32 azo-covalent organic polymer azo-COP-2 (109.6),33 benzimidazole-linked porous polymer BILP-2 (113),63 pyrrole-HCP (117),64 and Network-7 (119).65 We note that those MOP materials with high CO2/N2 selectivities often exhibited moderate CO2 uptakes (2−3 mmol g−1, 273 K, 1.0 bar, Table S4). In clear contrast, the current sample PCN-14 simultaneously possesses a high CO2 uptake of 5.33 mmol g−1, an ultrahigh CO2/N2 adsorption ratio (148), and a selectivity (156) at 273 K, which is very impressive in the studies of CO2 adsorption. Insight into the Adsorption Behavior. The excellent CO2 adsorption capacity over the PCN-14 sample can be assigned to the large surface area, high N content, and strictly ultramicroporous structure, which are all beneficial factors, as revealed by previous works.34−43 The normalized CO2 uptakes (273 K, 1.0 bar) per unit of surface area or N content are calculated and correlated to the corresponding surface areas and N contents, respectively (Figure S22), and the results show a general, although not exact, trend that the larger surface area and higher N content, the higher CO2 uptake (The details see the discussion text after Figure S22). Therefore, the PCN-14 sample simultaneously possessing large surface area and high N content presents the highest CO2 uptake, which is superior to not only the PCN-13 sample with almost the same N content but smaller surface area, but also other PCN-xy samples (e.g., PCN-64) with large surface areas but lower N contents. Therefore, the enriched heteroatoms (N, 20.10 wt % and S, 3.06 wt %) codoped in PCN-14 play the key roles in enhancing the high CO2 adsorption capacity, mainly due to the strong acid−base interaction between acidic CO2 molecules and basic N sites or SC functional groups, and the strong pole−pole interactions between the large quadrupole moment of CO2 molecules and polar sites associated with N- or S-functionalities.36−39,66 Besides, the CO2 uptake over PCN-14 with strictly ultramicroporous structure is also higher than those previous hierarchical carbon materials containing both large surface area and high N content.34,35,47 This observation agrees with the early finding that ultrafinely microporosity with high surface areas and narrow micropores smaller than 0.7 nm can provide more accommodations for trapping CO2 molecules via a volume-filling mechanism.40−43,67 Briefly, the strictly ultramicroporous structure of the PCN-14 sample with high surface area and N/S-co-doped polar surface deserves the high CO2 adsorption uptake under ambient pressure.

nonporous carbon, indicating the vital role of CN groups for the described consecutive trimerization and carbonization processes. Furthermore, although presently short of direct evidence, the well-defined crystal structure of the CIS precursor may possibly associate with the formation of the narrow ultramicropores, as suggested by a recently reported crystallized protic salt carbon precursor.57 CO2 Selective Adsorption. Figure 6, parts a and b, show the CO2 adsorption isotherms of all the synthesized PCN-xy series measured at 273 and 298 K, respectively. The corresponding CO2 adsorption capacities are listed in Table 1. All of these PCN-xy carbons show superior CO2 adsorption capacities (3.59−5.33 mmol g−1 at 273 K and 2.35−3.68 mmol g−1 at 298 K) at ambient pressure (1.0 bar). The highest CO2 uptakes, 5.33 mmol g−1 (23.5 wt %) at 273 K and 3.68 mmol g−1 (16.2 wt %) at 298 K, are achieved on the PCN-14 sample. These data are higher than all the reported CO2 uptakes over IL-derived porous carbon materials,47,49,51 and superior or comparable to those over the reported porous materials (typically, N-enriched porous organic polymer,58 MOP, or MOF-derived porous carbons15−19,31,59), but still inferior to those over some carbon adsorbents, including hard-templated carbons16,37 and physically or chemically activated carbons.13,14,17,38−40 In particular, at the low pressure 0.15 bar, PCN-14 exhibits very high CO2 adsorption capacities of 2.58 and 1.50 mmol g−1 at 273 and 298 K, respectively. The values are much higher than most of the reported porous carbons for CO2 capture under identical conditions,15,18,31,59−61 which is of great significance to broaden its potential application in separating CO2 from dilute gases.61 In addition, five-cycling reproducible CO2 sorption isotherms are observed (Figure S19), suggesting their good regeneration stability for CO2 capture. The N2 adsorption isotherms at 273 K for all the PCN-xy samples are measured to investigate their selective adsorption of CO2 versus N2, which is crucial for CO2 capture faced with various practical situations such as mixed gases and dilute CO2 in the flue gas. CO2/N2 selectivities of PCN-xy are estimated using initial slope ratios calculated from Henry’s law constants for single-component adsorption isotherms at low pressure coverage (Figure S20), which is one of the most common methods to calculate gas selectivities.32 The corresponding calculation results are described in Figure 6c. All of the samples exhibit very low N2 uptakes and give high CO2/N2 selectivities, which exactly relate to the textural properties and chemical compositions of these carbons. The highest value of 156 is observed over the PCN-14 sample (Figure 6c), mainly due to its high surface area (918 m2 g−1), narrow ultramicropore (0.59 nm), and high N content (20.10 wt %). Compared with PCN14, PCN-13 with a similar N content of 20.54 wt % and pore size (0.61 nm) but lower surface area (722 m2 g−1) gives a lower CO2/N2 selectivity of 125. The other carbon samples, including PCN-24, PCN-23, PCN-44, PCN-43, and PCN-64, present much lower CO2/N2 selectivities, ranging from 66 to 90, attributable to their lower N contents compared to those of PCN-14 and PCN-13. Although sample PCN-64 has the low N content of 7.04 wt %, it exhibits a considerable CO2/N2 selectivity of 74, which may arise from its large surface area and very small pore size of 0.51 nm. In particular, PCN-54 with abundant mesopores only gives a very low CO2/N2 selectivity of 29. Low N2 uptakes are observed over these PCN-xy samples even at enhanced pressure (273 K, 1.0 bar). For example, PCN14 presents an almost negligible N2 adsorption capacity of H



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CONCLUSIONS A new family of molecular level carbon precursors, cyanopyridinium crystalline dicationic salts (CISs), are task-specifically designed and prepared. Through straightforward carbonizing these molecular CISs at the relatively low temperature of 450 °C, series of highly N-enriched ultramicroporous carbons PCNxy with large surface areas are resulted. Neither additives nor pre/post-treatments are required in this process. The porosity and N content of the carbon samples can be precisely tailored by varying the intrinsic chemical structures and compositions of CISs. By virtue of the simultaneous high surface area (918 m2 g−1), N content (20.10 wt %), and narrow strictly ultramicroporosity (0.59 nm), the lead sample PCN-14 performs exceptionally high CO2 uptakes at 1.0 bar (5.33 mmol g−1 at 273 K and 3.68 mmol g−1 at 298 K), superior CO2/N2 selectivity (156 at 273 K), and the unprecedented adsorption ratio of CO2 to N2 (148 at 273 K and 1 bar). The strict ultramicroporosity and high N-doping content play crucial roles for both high CO2 adsorption capacity and high selectivity. This straightforward procedure changes the current situation of the inferior CO2/N2 selectivity over carbon materials, providing new insights into high-performance porous carbon materials for efficient selective CO2 capture.

The ultrahigh CO2/N2 selectivity over PCN-14 can be also assigned to the above-mentioned factors. Generally, gaseous separation selectivity is determined by the thermodynamic and kinetic discrepancy of different gases over the adsorbents. CO2philicity is the thermodynamic driving force for the selective adsorption of CO2 relative to N2. The high N content over the PCN-14 sample provides strong polar interaction between CO2 and lone-pair electrons on the N atom, enhancing the CO2philicity, which is however neutral toward N2. This discrepancy ultimately benefits the improvement of CO2/N2 selectivity.33 Similarly, S-doping also promotes the CO2-philicity but neutrals toward N2, thus further enhancing the CO2/N2 selectivity.66 However, the ultrahigh CO2/N2 selectivity over the PCN-14 sample cannot be merely attributed to the high Ndoping content because various other N-enriched carbon materials with large surface areas usually could not offer such a high selectivity (Table S4). In-depth structural analyses indicate that those previous N-enriched carbons all owned certain mesopores, while the PCN-14 sample is of strictly ultramicroporous structure. The narrow ultramicropores centered at 0.59 nm provide sufficiently large differentiation in kinetic diffusion between the small CO2 molecule (3.30 Å) and the relatively large one of N2 (3.64 Å), favoring the higher CO2/N2 adsorption selectivity.67 The strictly ultramicroporous structure excludes the possible decline of selectivity ascribed to mesoporosity. The contrastive data over the sample PCN-54 with hierarchical micro/mesoporous structure give another strong support for this speculation: They present the much inferior CO2/N2 selectivity of 29 (Figure 6c) compared to those over other PCN-xy samples with high ultramicroporosity. In a word, the strictly unique ultramicroporosity plays a critical role for both high adsorption capacity and high selectivity over PCN-xy samples. To further understand the adsorption results, the isosteric heats of adsorption (Qst) are calculated by the Clausius− Clapeyron equation from dual-site Langmuir−Freundlich fits of CO2 isotherms measured at 273 and 298 K. Figure 5d shows the plot of adsorption enthalpies as a function of the adsorbed CO2 uptakes. The high Qst values of CO2 (37−55 kJ mol−1) at low coverage are obtained over these PCN-xy samples, surpassing most of the reported values over porous carbons, and comparable to polyamine or imide-functionalized MOPs (e.g., PPN-6-CH2DETA, TPI-1@IC) (40−63 kJ mol−1).68,69 With the increase of CO2 uptake to 0.3 mmol g−1, the isosteric heats present sharp decline, and most of them keep constant at higher CO2 adsorbed uptakes. The saturated Qst values are retained at a high level (e.g., Qst 49.5 kJ mol−1 for PCN-14, Table S5), reflecting the strong CO2 affinity up to high CO2 loadings, which is consistent with the large CO2 adsorption capacities. The highest CO2 uptake happens over the PCN-14 sample with the highest initial and saturated Qst values. Such high adsorption enthalpies can be attributed to the pyridinic/ pyrrolic N and thiophene S enriched polar surface and strictly ultramicropore channels (0.59 nm), indeed reflecting the enhanced quadrupolar interaction between carbon framework and CO2 molecules, which accounts for the high CO2 uptake and CO2/N2 selectivity.67,69,70 Besides, the saturated Qst for PCN-14 still locates in the range of strong physisorption or weak chemisorption, causing a good trade-off between adsorption capacity/selectivity and reversibility.32

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b04842. 1 H NMR, 13C NMR, XRD, and TGA of CIS precursors. XRD, SEM, and FT-IR of CIS-derived N-doped carbons and their CO2 adsorption performance (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel: +86-25-83172264; fax: +86-25-83172261; e-mail: [email protected] (Y.Z.). *Tel: +86-25-83172264; fax: +86-25-83172261; e-mail: [email protected] (J.W.). Author Contributions

The manuscript was written through contributions of G.J.C., Y.Z., and J.W. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank greatly the National Natural Science Foundation of China (Nos. 21136005, 21303084, and 21476109), Jiangsu Provincial Science Foundation for Youths (No. BK20130921), Specialized Research Fund for the Doctoral Program of Higher Education (No. 20133221120002), and the Scientific Research and Innovation Project for College Graduates of Jiangsu Province (CXZZ13_0447).

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